Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the recording element, while un-imaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
It is well known in the art that printed images generated using ink jet technology are not very stable with regard to environmental effects of light and ozone. Another problem with such printed images is that they are easily scratched during handling, especially when pigmented inks are used, and are not resistant to typical aqueous solutions such as coffee, fruit punch, etc.
Several methods have been used to protect printed images generated using ink jet technology. One method is referred to as lamination and involves the application of a clear durable film over the printed image. Another method involves printing onto specially designed ink jet recording elements which are subsequently fused to form a clear durable film over the printing image. Both of these methods have drawbacks in that an integral or peripheral fusing station is required.
Another method for increasing the durability of printed images involves application of an overcoat composition onto the surface of the printed image using a brush, roller, sponge, etc. As the composition dries, a clear durable film is formed. This method is useful in a variety of commercial printing applications but is considered too impractical and undesirable for consumer use in the home. To this end, the use of a colorless ink supplied in a printhead of an ink jet printer has become increasingly popular. The printhead containing the colorless ink is typically part of the same carriage assembly containing colored inks, and the printer is instructed to jet the colorless ink either simultaneously with or after the colored inks are jetted.
U.S. Pat. No. 6,087,051; U.S. 2003/0193553 A1; U.S. 2003/0005945 A1; JP 2003-291484 A; JP 2000-225695 A; and JP 2002-144551 A describe methods of ink jet printing wherein a colorless ink is used to equalize gloss and/or provide image permanence. These references describe the use of polyurethanes in the colorless inks, but do not describe how the physical properties of the polyurethanes should be optimized in order to obtain jettability over extended periods of time, and at the same time, provide stain and durability for images printed on a variety of media.
U.S. 2002/0156153 A1 describes the use of polymeric additives in fixatives for ink jet printing in order to improve print quality and image permanence attributes on plain paper. This application discloses that a wide variety of polymers can be used, as long the polymer Tg, melting temperature, and molecular weight each fall into a particular range. The problem with this invention is that high amounts of organic solvents are needed for jetting, making the fixatives environmentally unfriendly and unsuitable for use on glossy ink jet recording elements.
U.S. Pat. No. 6,604,819 B2 relates to an ink jet image recording method that includes application of a solution having fine polymer particles during or after printing colored inks in order to provide weatherfastness. This patent states that the particles must have a weight average molecular weight of at least 100,000 such that a film is formed therefrom. This patent discloses the use of polyurethanes, but polyurethanes having a weight average molecular weight of at least 100,000 are not jettable using an thermal ink jet printhead.
U.S. 2002-0009547 A1 relates to a coating liquid for application to recorded images in order to provide image permanence, fixation and glossiness. This application discloses that fine polymer particles may be used in the coating liquid, and that it is especially desirable if the particles have an acid number of 100 or less.
Regardless of the above, there is a continuing need for inks and for protective coatings for images that have good jetting and printer reliability performance as well as stain resistance and rub resistance.